Gas discharge ultraviolet lasers used as a light source for integrated circuit lithography typically are line narrowed. A preferred line narrowing prior art technique is to use a grating based line narrowing unit, called a line narrowing package or xe2x80x9cLNPxe2x80x9d, along with an output coupler to form the laser resonance cavity. These systems also include a wavemeter in which laser pulse energy and wavelength are measured. The gain medium within this cavity is produced by electrical discharges (produced by a pulse power system) into a circulating laser gas such as krypton, fluorine and neon (for a KrF laser); argon, fluorine and neon (for an ArF laser); or fluorine and helium and/or neon (for an F2 laser). Discharges in these lasers are produced by high voltage pulses with peak voltages in the range of between about 15,000 volts to 30,000 volts. A typical prior art excimer laser is described in U.S. Pat. No. 6,128,323 which is incorporated herein by reference.
It is known to provide nitrogen purges to selected optical and high voltage components of these laser systems. The optical components including the LNP, the output coupler and the wavemeter are purged primarily to prevent damage to the optical components caused by the interaction of oxygen or other airborne contaminants with the components in the presence of ultraviolet radiation. High voltage components are purged to prevent flashovers which can occur in the presence of air, especially air containing contaminants. The purged high voltage components include (1) a high voltage cable connecting the portion of the pulse power system called the commutator to another portion called the compression head and (2) the high voltage components mounted on top of the laser chamber which includes a bank of capacitors which accumulate the discharge pulse energy and the additional electrical components within the compression head all of which operate at voltages in excess of about 15,000 volts at the electrical peak of each pulse.
These purge systems are important for KrF lasers which produce a laser beam at a wavelength about 248 nm but they are even more important for ArF lasers and F2 lasers which produce much more energetic beams and operate at higher discharge voltages.
The N2 purge systems typically used in prior art excimer laser systems consists of an N2 line which directs flowing purge gas to a chamber containing the components being purged. The N2 merely floods the chamber and exits through miscellaneous openings. Only in particular circumstances is it normal practice even to provide a specific outlet port and when an outlet port is provided, the chamber is typically not sealed so that the N2 may exit various miscellaneous openings.
When a laser system is started, upon initial installation or after a service procedure, the purge flow must be established for a certain period of time before light output begins; this allows a clean, oxygen-free environment to be established within the optical modules. One conventional way to achieve this is to have the laser control system enforce a predetermined xe2x80x9ctimeoutxe2x80x9d period whenever the laser power is first turned on. This may not be desirable, since the power interruption does not necessarily mean that the purge system integrity has been compromised. In addition, some service procedures could disturb the purge system integrity without interrupting the laser power or initiating a timeout sequence. Excimer lasers are often used in semiconductor processing facilities, where substantial operating losses can occur during equipment idle time. Thus, there is a strong desire to maximize the useable operation time of the laser system, and eliminate any unnecessary wait periods. In this specification and the claims we will use the phrase xe2x80x9claser timeoutxe2x80x9d to refer to time periods when the laser is unavailable to produce laser light due to lack of purge flow or uncertainty regarding the laser purge equipment.
What is needed is a system for maintaining and monitoring the inert purge gas flow through a laser system, regardless of the state of the power applied to the laser. Such a purge system would allow resumption of laser operations as soon as is consistent with protection of optical components.
The present invention provides a laser component purge system for discharge lasers. The LNP, the output coupler and the wavemeter are contained in sealed chambers each having a purge inlet port and a purge outlet port. Purge gas such as N2 is directed to each of the inlet ports. A purge monitoring system is provided which monitors the purge flow and provides one or more signals to a processor which is programmed to minimize laser timeouts attributable to purge conditions without endangering the purged optical components. In a preferred embodiment, gas exiting the outlet ports are directed to flow monitors which provide the one or more signals to the processor. Purge gas may be exhausted or recirculated.